From the U.S. Pat. No. 4,742,808, a circuit for measuring the internal resistance of a lambda probe is known which exhibits a load resistor which is selectively connected in parallel with the probe by means of a switch. A means for measuring the probe voltage measures the probe voltage in the loaded and unloaded state. From the two voltage values and the resistance value of the load resistor, the internal resistance is calculated. As soon as the internal resistance has dropped below a predetermined threshold value, the voltage of the unloaded probe is used for controlling purposes.
For all circuits by means of which the voltage of a lambda probe is detected, the internal resistance of the probe can be used, when the probe is sufficiently warm, to set up a relationship between the measured voltage and the voltage which would actually be measured with a lambda value present in each case when the internal resistance would correspond to that of an operationally warm probe. Thus, the measured voltage can be corrected by means of the internal resistance. It is problematic that when the lambda probe heats up, the internal resistance changes by several powers of ten, namely from a few megohms at 200.degree. C. down to less than 100 ohm at 800.degree. C. If the above-mentioned circuit is to be used to measure the internal resistance in this entire range of resistance values, it is necessary to use several load resistors having quite different resistance values. This is because the order of magnitude of the value of the load resistor must correspond to the internal resistance in order to be able to determine, firstly, when the probe is sufficiently operationally warm and thereafter to obtain reliable measurement results. This leads to the internal resistance first having to be measured roughly by loading it with an arbitrarily selected load resistance. Once the internal resistance has been estimated by means of this measurement, a load resistance must be selected which has a resistance value which is of the order of magnitude of the internal resistance. It is only then that the internal resistance can be reliably measured. As a result, it is necessary during temperature changes of the probe, that is particularly during warmup, to switch time and again from one load resistor to another. The circuit configuration and the measuring process which can be carried out therewith are thus complicated.
The internal resistance of a probe can be measured without any direct-voltage loading within a narrow resistance range by means of a circuit which is described in U.S. Pat. No. 4,419,190. An alternating current of known intensity is sent through the probe and the alternating voltage dropped across the probe is measured. The alternating voltage component is separated from the direct voltage component of the probe and the internal resistance is determined from alternating current and alternating voltage. The measuring process which can be carried out by means of this circuit is thus simple but the circuit complexity is also considerable.
The invention is based on the object of specifying a simple circuit for measuring the internal resistance of a lambda probe.